Influenza is a viral infectious disease that repeatedly becomes epidemic with different antigen every year. The vaccines approved in Japan for the prophylaxis of influenza are those for subcutaneous administration. Since the vaccination induces an IgG antibody having a neutralizing activity in the serum, it is highly effective for preventing progression of the condition into a more severe one such as pneumonia and the like. In the upper airway mucosa, which is the infection site, however, IgA is the main prophylactic component. Since IgA is not induced by subcutaneous administration, the infection-preventive effect is not sufficient. Therefore, the development of an infection-preventive vaccine has been desired for a long time.
The present inventors have developed an influenza vaccine for mucosal administration comprising a double-stranded RNA and an influenza virus antigen, which is superior in the infection-preventive effect (WO2005/014038).